As is well known, the performance and bandwidth of fiber-optic communication systems are limited by chromatic dispersion, whereby different frequencies propagate at different speeds along the fiber-optic path. Authors G. H. Smith, D. Novak, and Z. Ahmed teach “A Technique for optical SSB generation to overcome dispersion penalties in fibre-radio systems,” in the Electronic Letters at volume 33, at number 1, in 1997. Authors K. Yonenaga and N. Takachio teach a “A fiber chromatic dispersion compensation technique with an optical SSB transmission in optical homodyne detection systems,” in the IEEE Photonics Technology Letters, at volume 5, in pages 949-951, in 1993. Dispersion in analog optical links creates frequency transmission nulls, which are known as dispersion penalties, that cause a loss of information, usually shown as dispersive nulls. This dispersive nulling effect is caused by the deconstructive interference between the upper and lower sidebands of a double sideband (DSB) modulated signal. This effect is at its worst when the upper and lower sidebands are exactly 180 degrees out of phase with each other, completely annihilating the signal. A well-known method to avoid these dispersive penalties is to use only single sideband (SSB) modulation. In the SSB approach, one of the two frequency sidebands produced during DSB modulation is eliminated. Typically, optical SSB modulation is realized by an RF phase shifting technique, which at the present level of technology, has a bandwidth limitation around 18 GHz.
There are two primary methods to achieve optical SSB modulation, including the phase discrimination method and the optical filtering method. The phase discrimination method requires a RF hybrid coupler and balanced optical modulator. The bandwidth capability of available RF hybrid couplers, however, is currently disadvantageously limited to an upper range of 18 GHz. In the optical filtering method, optical filtering is realized by an optical bandpass filter to remove either the upper or lower sideband of the RF signal. This optical filtering method, however, is possible only when the optical carrier frequency is fixed and stable.
In many applications, such as time-stretched photonic analog-to-digital converters, the optical carrier frequency is swept, that is chirped, by linearly varying carrier frequency in time. Authors Y. Han and B. Jalali, teach a “Photonic Time-Stretched Analog-to-Digital Converter,” in the Journal of Lightwave Technology, in volume 12, at pages 3085-3103, in The time-stretched photonic analog-to-digital converter may include an input D1 dispersive element to provide a chirped carrier, an electrical-to-optical modulator modulating a chirped carrier by a baseband signal communicated along an optical fiber function as a second D2 dispersive element. The optical signal is then communicated along the optical fiber to an optical-to-electrical photodetector. The time-stretched photonic analog-to-digital converter functions as an optical preprocessor whereby an RF signal is stretched by a factor M=(D1+D2)/D1. Upon optical-to-electrical conversion, the time-stretched photonic analog-to-digital converters increase the effective bandwidth of an electronic analog-to-digital converter. For example, a 4 GHz bandwidth analog-to-digital converter combined with a time-stretch preprocessor of M=250 will result in a 100 GHz bandwidth analog-to-digital converter. Under normal operating conditions, the preprocessor will suffer a frequency fading penalty caused by the double sideband signals. However, there is currently no broadband SSB modulation solution for chirped optical signals. This SSB signal format is preferred over traditional double sideband modulated signals in order to avoid frequency fading and information loss in the optical fiber. Electrical and optical processing systems have long used compressors for translating frequency domain signals into time domain signals. Electrical and optical processing systems have long used expanders for translating time domain signals into frequency domain signals. Such compressors and expanders are typically simple dispersive elements such as dispersive transmission lines or chirped gratings. An expander could be a portion of an optical fiber. However, compressing and expanding in the frequency domain does not remove a sideband from double sideband signals, and hence, are unsuitable in tandem for communicating chirped modulated double sideband signals without dispersive nulling.
The Mach-Zehnder modulator is an optical modulator widely used in the telecommunications industry to generate digital waveforms, the most common being on-off keying. The Mach-Zehnder modulator device may use waveguide interferometers with electrodes used to impart path length changes. The path length changes can be selected to cause total constructive and destructive interference at the output port, or full on or off operations. Mach-Zehnder modulators are routinely used in 40 Gbps links and some have been demonstrated with bandwidths capable of supporting 100 GHz modulation. Typical Mach-Zehnder modulators exhibit extinction ratios of 100:1 or better. Mach-Zehnder modulators offer ultra high frequency modulations. Mach-Zehnder modulators have been used in optical systems.
Compressing a modulated chirped optical carrier in time produces a waveform, which mirrors the shape of the RF frequency spectrum. Previously, this known compression property has been used to measure RF spectra in the time domain. In a well understood phenomenon, analogous to spatial diffraction, a convolution occurs between the transform-limited pulse and the RF frequency spectrum of the input signal. Authors R. Saperstein, D. Panasenko, and Y. Fainman, teach a “Demonstration of a microwave spectrum analyzer based on time-domain optical processing in fiber,” in Optics Letters, at volume 29, at pages 501-503, in 2004. Also, chirped Bragg gratings have been used to process chirped signals but suffer from fabrication introduced phase error such as residual group delay ripple, which in turn disadvantageously distorts the signal. Author R. Kashyap teaches “Fiber Bragg Gratings,” in the Academic Press, of San Diego, in 1999. However, fiber Bragg gratings do not inherently filter sidebands. Communication of chirp optical signals along optical fibers disadvantageously produces dispersive nulling. These and other disadvantages are solved or reduced using the invention.